Method for producing ceramics reinforced with three-dimensional fibers

ABSTRACT

A method for producing ceramics reinforced with three-dimensional fibers, which entails: a) setting a filter plate in contact with one surface of a three-dimensional fiber preform and a porous metallic plate in contact with said filter plate; b) setting a slurry in contact with a surface of the preform opposite to the one surface; c) exerting pressure on the slurry and, at the same time, subjecting the preform to vacuum aspiration through the filter plate, thereby obtaining a composite having an impregnated preform, the impregnation being effected in a single step; d) heating the composite; and e) firing the resultant composite.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the production of ceramicsreinforced with three-dimensional fibers and an apparatus for theproduction of the ceramics.

2. Description of the Background

Ceramics reinforced with fibers have recently been attracting attentionas products improved by elimination of such drawbacks as low toughnessand poor reliability of strength which are monolithic ceramics. Threekinds of fiber reinforcing type are known: one-dimensional reinforcementin which fibers are laid out only in one direction, two-dimensionalreinforcement in which fibers are laid out in two directions (in aplane), and three-dimensional reinforcement in which fibers are laid outin three directions (in a cubic pattern). These kinds of fiberreinforcement manifest the following characteristics.

The one-dimensional fiber reinforcing type manifest tensile and flexuralstrength sufficiently in the direction of the fibers but only meagerlyin directions at or near perpendicular to the direction of the fiberreinforcement. The two-dimensional fiber reinforcing type exhibits greatstrength in the plane containing the fibers but poor strength in thedirection perpendicular to the plane. They also tend to induce peeling.In contrast, the three dimensional fiber reinforcing type shows strengthvirtually uniformly in an isotropic manner in all directions. Thethree-dimensional fiber reinforcing type is therefore ideal for thepurpose of reinforcement.

While fiber-reinforced ceramics using one-dimensional andtwo-dimensional fiber reinforcements can be manufactured rather easily,those using three-dimensional fibers cannot.

Basically, the ceramics reinforced with three-dimensional fibers(hereinafter referred to as "three-dimensional fiber-reinforcedceramics") have a construction comprising a three-dimensionally formedfibrous web (preform) and a ceramic mass filling the interstices betweenthe component fibers of the fibrous web to give rise to a matrix. Sincethe three-dimensional fibrous preform, unlike the one-dimensional andtwo-dimensional fibrous preforms, has component fibers densely andcubically interwoven, the individual fibers thereof strongly contact oneanother and form narrow interstices and not all of the gaps between theadjacent fibers are interconnected. Thus, the three-dimensional fibrouspreform is not easily filled with a matrix substance.

The filling of the three-dimensional fibrous preform with the matrixsubstance is generally effected by mixing the matrix substance with suchadditives as an organic binding agent to form a liquid substance,injecting this liquid substance into a preform, and then firing theresultant composite. In this case, the organic binding agent and otheradditives contained in the liquid substance filling the preform inducethermal decomposition and the like and, as illustrated in FIG. 1, giverise to many pores 4 in a matrix part 3. For the purpose of producingceramics which are reinforced with densely interwoven three-dimensionalfibers, therefore, it is necessary that the number of such pores in thecomposite should be decreased to the fullest possible extent.

This invention has for its object the provision of an improved methodfor the production of ceramics reinforced with three-dimensional fibersand an apparatus for working this method.

SUMMARY OF THE INVENTION

To accomplish the object described above, this invention provides amethod for the production of ceramics reinforced with three-dimensionalfibers, which method comprises setting a filter plate on one surface ofa three-dimensional fibrous preform, superposing on the surface of thethree-dimensional fibrous preform opposite to the surface thereofcovered by the filter sheet a slurry consisting of a fine ceramicpowder, a precursory substance of the ceramic mentioned above, and asolvent, exerting high pressure on the slurry and reducing the pressureon the filter sheet side for causing the slurry to permeate thethree-dimensional fibrous preform and enabling the solvent in the slurryto penetrate the filter sheet and pass to the exterior thereof throughaspiration, consequently obtaining a composite having the preform filledwith the fine ceramic powder and the finely divided precursorysubstance, heating the composite for thermally decomposing theprecursory substance, and thereafter firing the resultant composite. Theinvention also provides an apparatus for the production ofthree-dimensional fiber-reinforced ceramics, which apparatus comprises afoundation open on its upper side and enclosing an empty space and acylinder disposed on the foundation, the empty space of the foundationand the open space of the cylinder integrally forming a continuous emptyspace, and the continuous empty space having an empty region at thelowermost part thereof for allowing a filter plate to be set fasttherein and permitting a pressure piston to be set fast above the filterplate to form between the pressure piston and the filter plate an emptyregion for accommodating a slurry for the production ofthree-dimensional fiber-reinforced ceramics.

The method mentioned above embraces the case of using a slurry whichfurther comprises an amorphous ceramic powder obtained by thermallydecomposing the precursory substance of the ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the occurrence of pores in a ceramicproduced by filling the interstices of a three-dimensional fibrouspreform with a matrix substance.

FIG. 2 is a diagram illustrating an apparatus to be used for theproduction of three-dimensional fiber-reinforced ceramic material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a fine ceramic powder using SiC, a fine β-SiC powder (having aparticle diameter of 0.3 μm, for example) proves ideal. Si₃ N₄ is alsouseful.

The fine powder under discussion generally is desired to have a particlediameter in the range of 0.01 to 3 μm. If the particle diameter is lessthan 0.01 μm, the individual particles of the powder undergo conspicuousagglomeration. If the particle diameter exceeds 3 μm, the powder cannoteasily enter into the interstices between the adjacent fibers. The finepowder may contain therein not more than 10% by weight of sintering anddispersing auxiliaries.

The solvent to be used in the slurry is not particularly restricted.Practically, toluene and xylene are advantageously usable herein.

The solvent may contain an organic binding agent. As concrete examplesof organic binding agents advantageously usable herein, polyvinylalcohol and hydrocarbon type waxes may be cited. The upper limit of thecontent of the organic binding agent in the solvent is 10% by weight.

As concrete examples of precursory substances favorably usable herein,polycarbosilane and polysilastyrene, which are suitable for ceramics ofSiC, and polysilazine and polyhydrosilazine, which are suitable forceramics of Si₃ N₄, may be cited.

In the firing step of process of manufacturing a ceramic composite, theorganic binding agent and the precursory substance in the slurrythermally decompose and give off volatile components. Pores develop inthe matrix part of the composite material (FIG. 1) unless the volatilecomponents are removed to the fullest possible extent. For the purposeof repressing the occurrence of such pores and ensuring production of ahighly densified composite material, therefore, it is necessary toreduce the amount of the organic binding agent and the precursorysubstance in the slurry to the minimum required.

One of the main roles of the organic binding agent and the precursorysubstance used in the present invention resides in enabling the slurryto permeate the three dimensional fibrous preform, thereby increasingthe interfacial strength between the fibers and the filling substancesin a formed green article manufactured in a dry state, and facilitatingthe subsequent processes. The product of decomposition of the precursorysubstance serves as a sintering auxiliary. The slurry substantiallyconsists of the ceramic powder, the organic silicon resin as aprecursory substance, and the solvent.

The ceramic powder content of the slurry is in the range of 70 to 90% byweight and the organic silicon resin content thereof is in the range of30 to 10% by weight. The slurry concentration is in the range of 100 to200 g/liter. These proportions are factors to be determined empirically.The precursory substance generally undergoes thermal decomposition inthe process of firing and consequently converts to ceramic. Theproportion of this precursory substance which yields to this conversionis about 70% at most.

The slurry, by exertion of high pressure thereon, is caused to permeatethe fibrous preform which is obtained by three-dimensionallyinterweaving carbon fibers, silicon nitride fibers, or glass fibers.Simultaneously with the injection of the slurry, the three-dimensionalfibrous preform is subjected to vacuum aspiration through a paper filterand a sintered porous metallic plate which are superposed on theopposite side of the preform so as to curb passage therethrough of theceramic powder, organic binding agent, and precursory substance andallow positive passage of the solvent part therefrom.

The method just mentioned is aimed at purging the solvent part from thethree-dimensional fibrous preform while minimizing discharge of theother parts therefrom. Actually, in the case of the slurry answering thedescription given above, a fair amount, specifically about 40 to 50% byweight, of the other parts are discharged from the preform. It isimportant from the practical point of view to decrease the amount ofthis discharge.

The substances which are produced by the decomposition of the fineceramic powder contained in the slurry and the precursory substanceincorporated in advance in the slurry are invariably crystallinemorphologically. The portion of the precursory substance which isdischarged from the preform in combination with the solvent part can becompensated for by using a slurry which additionally incorporatestherein an amorphous ceramic substance (obtained by the decomposition ofthe precursory substance of the ceramic prior to the addition of theslurry).

The amount of the amorphous ceramic substance to be added for thepurpose just mentioned is only required to be such as to replace 5 to10% of the fine ceramic powder already existing in the slurry. Sincethis substance serves as a sintering material, excessive additionthereof is undesirable.

When the matrix substance remaining after the discharge of the solvent,namely the three-dimensional fibrous preform impregnated with the fineceramic powder, the precursory substance, and an amorphous ceramicsubstance obtained by the thermal decomposition of the precursorysubstance, is heated, the precursory substance undergoes thermaldecomposition. A three-dimensional fiber-reinforced ceramic is obtainedby sintering the product of this thermal decomposition. The temperatureof the thermal decomposition is in the range of 600° to 1,000° C. andthe temperature of the sintering in the range of 1,500° to 2,000° C.

FIG. 2 is a schematic diagram of an apparatus according to thisinvention to be used for filling the interstices between the componentfibers of a three-dimensional fibrous preform.

This apparatus is provided with a foundation 11 having an aspirationchamber 12 formed therein by depression and a cylinder 13 set in placeon the foundation 11 and provided with a cylinder hole 14 communicatingwith the aspiration chamber 12. The foundation 11 is so adapted that theaspiration chamber 12 communicates with a vacuum pump through anaspiration path 15. A porous stainless steel bottom plate 16 is mountedon the aspiration chamber 12 through the medium of an O-ring 17. Aporous sintered metallic plate (Porouston) 18 is mounted on the bottomplate 16. A paper filter 19 is laid on the sintered metallic plate 18.An O-ring 20 is interposed between the paper filter 19 and the cylinder13 for preventing leakage of high pressure slurry (to be describedlater) from the periphery of the paper filter 19.

The cylinder hole 14 includes an empty space for accommodating athree-dimensional fibrous preform 5 and a slurry 6 on top of the paperfilter 19. A piston 21 is set in place over this empty space. The piston21 is actuated by a ram 22 of a pressing device (not shown). An O-ring23 and as many piston rings 24 of Teflon (trademark) as are required areset in place around the periphery of the piston 21 so as to precludeotherwise possible leakage of the slurry 6 through the gap between theperiphery of the piston 21 and the inner surface of the cylinder 13.

When the space accommodating the three-dimensional fibrous preform 5 andthe slurry 6 is closed on the upper side thereof with the piston 21, airis entrapped in the empty space overlying the slurry 6. For the purposeof discharging this entrapped air, an air vent 26 is bored through thepiston 21 and a spherical steel plug 27 for closing the air vent 26 isinserted in the inner terminal of the air vent 26 and kept in a closedstate by means of a feed screw 28 which is screwed into the air vent 26.

The stainless steel bottom plate 16 mentioned above is formed with alarge number of holes measuring approximately 1 mm in diameter. Thepaper filter 19 has pores not more than 0.3 μm in diameter formedtherein for the purpose of precluding passage therethrough of a fillerpowder (generally not more than 1 μm in diameter) contained in theslurry 6 and allowing exclusive passage therethrough of the solventcomponent.

It has been experimentally established that in the apparatus of thisinvention constructed as described above, the slurry 6 held inside thecylinder hole 14 does not ooze out even under as large a pressure as 400kgf/cm² exerted on the piston 31. For the purpose of enabling the finepowder, organic binding agent, and precursory substance contained in theslurry 6 to be packed uniformly at a high density in the fibrous preform5, the pressing speed of the pressing device can be freely set in therange of 0.05 to 5 mm/min. The pressing device is desired to be capableof accurately controlling this pressing speed (within 5 mm/min±0.2% inthe case of uniform speed control and in the range of 1.00 to 20.00 tonforce±1% in the case of fixed load control).

Preparatory to the production of the three-dimensional fiber-reinforcedcomposite material by the use of the apparatus for productionconstructed as described above, the bottom plate 16, the sintered plate18, and the paper filter 19 are set in place inside the cylinder hole14, the fibrous preform 5 is placed thereon, and the slurry 6 or theliquid matrix substance is placed thereon in an amount greater than thebulk volume of the fibrous preform 5. Then, the spherical feed screw 28of steel in the piston 21 is loosened to open the spherical steel plug27, the entrapped air on the surface of the slurry 6 is discharged, thespherical steel plug 27 is closed, the piston 21 is lowered to depressthe slurry 6 downwardly, and the interior of the aspiration chamber 12is aspirated from below by means of a vacuum pump. As a result, theslurry 6 is packed densely in the interstices between the individualfibers of the fibrous preform 5.

In this case, since the O-ring 23 and as many piston rings 24 asnecessary are set in place around the periphery of the piston 21, theslurry 6 does not ooze out through the gap between the periphery of thepiston 21 and the inner surface of the cylinder 13 even when the piston21 is lowered at a high pressure. Moreover, in the bottom part of thecylinder hole 14, the otherwise possible leakage of the slurry of highpressure is precluded because the O-rings 20, 17 are set in placerespectively above the paper filter 19 and below the bottom plate 16.The paper filter 19 fulfills the function of obstructing passagetherethrough of the filler powder in the slurry and permitting exclusivepassage therethrough of the solvent part of the slurry. The solvent partpassing through the paper filter 19 is discharged into the aspirationchamber 12 via the holes in the porous sintered metallic plate 18 andthe bottom plate 16 which both underlie the paper filter 19.

The combination of the pressure exerted by the piston and the aspirationcaused by the vacuum pump brings about the following benefits.

1. During the pressing operation which is effected by placing thefibrous preform 5 and the slurry 6 in the cylinder 13 and subsequentlyinserting the piston 21 into the cylinder 13, the air entrapped in thefibrous preform 5 and between the upper surface of the slurry 6 and thepiston 21 is easily aspirated by the operation of the vacuum pump and,as a result, the matrix can be packed with high density.

2. Prior to the pressing operation, the slurry 6 placed in the cylinder13 is in a state of low viscosity (because of low concentration of theorganic binding agent or the precursory substance in the liquid medium)and therefore can easily enter the gaps in the fibrous preform 5.Between the pressing-aspirating operations produced by the apparatus ofFIG. 2, the aspirating operation permits particularly copious separationof the solvent part in the slurry 6 and, as a result, the solute (theorganic binding agent or precursory substance) of high concentration canbe packed together with the ceramic powder in the gaps of the fibrouspreform 5.

This effect can be adjusted by suitably choosing the diameter of thepores formed in the paper filter 19 disposed in the bottom part of thecylinder 13 of FIG. 2. Generally, when the paper filter has pores of asmall diameter (such as, for example, 0.3 to 0.1 μm), the solute partcan be retained in a fairly large amount in the slurry.

3. The combination of the pressing operation with the aspiratingoperation curtails the time required for the removal of the solvent fromthe slurry.

Now, concrete examples of the method for the production of a compositematerial according with this invention will be described below.

Three-dimensional fiber-reinforced ceramic were produced by the methodof this invention using the apparatus of this invention constructed asillustrated in FIG. 2.

EXAMPLE 1

As a three-dimensional fiber preform, a section 45 mm×60 mm cut from afibrous mass (fiber content 54%) obtained by perpendicularlyinterweaving carbon fibers in the form of a flat plate 6 mm in thicknesswas used.

A slurry to be injected into this three-dimensional fibrous preform wasprepared by combining 80 g of a fine SiC powder (average particlediameter 0.3 μm) with 20 g of polysilastyrene as a precursory substance,adding 5 g of an aluminum boride powder to the resultant blend, andthoroughly mixing the blend with 100 g of toluene added thereto as anorganic solvent.

The three-dimensional fibrous preform of carbon fibers and the slurrywere placed in the cylinder, pressed to a maximum pressure of 24 MPa bylowering the piston at a pressing speed of 0.05 mm/min, and leftstanding under this pressure for 1.5 hours to produce a formed mass of afiber-ceramic precursor composite.

Then, the formed mass of composite was heated in an atmosphere of argonat 700° C. to effect thermal decomposition of the precursory substanceand fired in an atmosphere of argon at 2,000° C. for about one hour togive birth to a three-dimensional fiber-reinforced SiC compositematerial having a very small pore content (15% in porosity).

EXAMPLE 2

A concrete example of the method of this invention for the production ofa composite material will now be described.

In an atmosphere of N₂, 23.1 g of polysilazane as a precursory substancewas thermally decomposed by heating to 850° C. and pulverized to obtaina fine amorphous powder of Si₃ N₄. Then, a slurry was prepared byplacing 15 g of this fine powder, 80 g of an α-Si₃ N₄ powder (containingAl₂ O₃ and Y₂ O₃ as sintering auxiliaries), and 7 g of polysilazane intoluene and thoroughly mixing the resultant blend with a ball millmixer. This slurry was placed together with a plate-shapedthree-dimensional fibrous preform (having a fiber content of 54% andmeasuring 45 mm×60 mm×6 mm thick) obtained by interweaving carbon fibersin the cylinder of the apparatus for the production of a compositematerial illustrated in FIG. 1 and pressed in the cylinder to a pressureof 30 MPa to effect impregnation of the preform with the slurry. As aresult, a fiber/matrix composite green body was produced.

Thereafter, the body was heated at 850° C. in an atmosphere of N₂ toeffect thermal decomposition of the polysilazane and further sintered inthe same atmosphere at 1,650° C. to produce a composite material. Thebulk density of this composite material was 1.55 g/cm². The matrix wasfound to comprise βSi₃ N₄ as the main phase and a small amount of α-Si₃N₄ phase.

When the matrix substance was packed in the gaps of thethree-dimensional fibrous preform by the process described above, thesubstance other than the solvent could be precluded from passing throughthe paper filter to a high degree.

What is claimed is:
 1. A method for the production of ceramicsreinforced with three-dimensional fibers, which comprises:a) setting afilter plate having pores of not more than about 0.3 μm in diameter incontact with one surface of a three-dimensional fiber preform and aporous metallic plate in contact with said filter plate; b) setting aslurry in contact with a surface of said preform opposite to said onesurface, said slurry having a concentration within a range of about 100to 200 g/l and consisting of about 70 to 90% by weight of a fine ceramicpowder having a particle diameter in a range of about 0.01 to 3 μm, andabout 30 to 10% by weight of an organic silicon resin which is a ceramicprecursor, in a solvent; c) exerting a pressure of not less than about400 kgf/cm² on said slurry at a pressing speed in a range of about 0.05to 5 mm/min, at the same time, subjecting said preform to vacuumaspiration through said filter plate, thereby obtaining a compositehaving said preform impregnated with said fine ceramic powder and saidorganic silicon resin, said impregnation of said preform being effectedin a single step; d) heating said composite at a temperature in a rangeof about 600° to 1,000° C. to thermally decompose said organic siliconresin; and e) firing the resultant composite at a temperature in a rangeof about 1,500° to 2,000° C.
 2. A method according to claim 1, whereinsaid fine ceramic powder contains not more than 10% by weight ofauxiliaries.
 3. A method according to claim 1, wherein said solventcontains an organic binding agent.
 4. A method according to claim 3,wherein said organic binding agent is at least one member selected fromthe group consisting of polyvinyl alcohol and hydrocarbon waxes.
 5. Amethod according to claim 1, wherein said solvent is at least one memberselected from the group consisting of toluene and xylene.
 6. A methodaccording to claim 1, wherein said ceramic fibers are at least onemember selected from the group consisting of carbon fibers, siliconcarbide fibers, silicon nitride fibers, and glass fibers.
 7. A methodaccording to claim 1, wherein said ceramic substance is SiC.
 8. A methodaccording to claim 1, wherein said organic silicon resin is at least onemember selected from the group consisting of polysilastyrene andpolycarbosilane.
 9. A method according to claim 8, wherein said organicsilicon resin is at least one member selected from the group consistingof polysilazane and polyhydrosilane.
 10. A method according to claim 1,wherein said ceramic substance is Si₃ N₄.
 11. A method according toclaim 1, wherein said slurry further comprises a fine amorphous ceramicpowder obtained by thermally decomposing a ceramic precursory substance.12. The method according to claim 1, wherein said fine amorphous ceramicpowder replaces said fine ceramic powder present in said slurry and isin an amount of about 5 to 10% by weight of said fine ceramic powder insaid slurry.
 13. The method according to claim 1, wherein the pressureexerted on said slurry is not more than about 30 MPa.